Ca2+ and Mg2+-binding and a putative calmodulin type Ca2+-binding site in Synechococcus Photosystem II

Thylakoids and Photosystem II particles prepared from the cyanobacterium Synechococcus PCC 7942 washed with a HEPES/glycerol buffer exhibited low rates of light-induced oxygen evolution. Addition of either Ca²⁺ or Mg²⁺ to both thylakoids and Photosystem II particles increased oxygen evolution independently, maximal rates being obtained by addition of both ions. If either preparation was washed with NaCl, light induced O₂ evolution was completely inhibited, but re-activated in the same manner by Ca²⁺ and Mg²⁺ but to a lower level. In the presence of Mg²⁺, the reactivation of O₂ evolution by Ca²⁺ allowed sigmoid kinetics, implying co-operative binding. The results are interpreted as indicating that not only Ca²⁺, but also Mg²⁺, is essential for light-induced oxygen evolution in thylakoids and Photosystem II particles from Synechococcus PC 7942. The significance of the reactivation kinetics is discussed. Reactivation by Ca²⁺ was inhibited by antibodies to mammalian calmodulin, indicating that the binding site in Photosystem II may be analogous to that of this protein.Abbreviation HEPES n-2-Hydroxyethylpiperazine--2-ethane sulphonic acid     

Fluorescence and Fourier-transform infrared spectroscopic studies on the role of disulfide bond in the calcium binding in the 33 kDa protein of Photosystem II

The 33 kDa protein of Photosystem II has one intrachain disulfide bond. Fluorescence spectroscopy shows that the major groups in the protein that bind to Ca²⁺ should be the carboxylic side groups of glutamic acid and/or aspartic acid. Fluorescence and Fourier-transform infrared (FTIR) spectroscopic studies indicate that the conformation of the 33 kDa protein is altered upon reduction, while the reduced protein still retains the secondary structure. FTIR spectroscopy also shows that the metal ions induce a relative decrease of unordered structure and -sheet, and a substantial increase of -helix in both the intact and the reduced 33 kDa protein. This indicates that the addition of cations results in a much more compact structure and that both the intact and the reduced 33 kDa proteins have the ability to bind calcium. The above results may suggest that the disulfide bridge is not essential for calcium binding.Abbreviations CD circular dichroism - FTIR Fourier transform infrared - La lanthanum - PS photosystem - Tb terbium     

Base cation biogeochemistry and weathering under oak and pine: a controlled long-term experiment

Large earthen-walled lysimeters at the San Dimas Experimental Forest in southern California present a unique opportunity to assess vegetation effects on biogeochemical processes and cation release by weathering in controlled soil-vegetation systems where archived samples of soil parent material are available for comparison. The lysimeters were filled in 1937 with homogenized fine sandy loam derived on site from the weathering of diorite, and planted in 1946 with scrub oak (Quercus dumosa) and Coulter pine (Pinus coulteri). Changes in base cation contents were measured in above-ground biomass, and total and exchangeable soil pools to a depth of 1 meter. All cations in the non-exchangeable soil pool decreased relative to the initial fill material, indicating release by weathering. Sodium and K were depleted from both exchangeable and non-exchangeable pools of the soils. Plant uptake of Na was minimal, whereas K storage in vegetation exceeded the loss from the exchangeable soil pool. In both soil-vegetation systems, but especially for oak, there was an increase in exchangeable Ca and Mg. For all base cations, storage in above-ground biomass was greater for oak, whereas losses by weathering from the non-exchangeable soil pool were greater under pine. Strong evidence supports biocycling as a controlling mechanism resulting in greater Ca and Mg release by weathering under pine. In addition, decreases in non-exchangeable Ca and Mg were strongly correlated to decrease in Si under oak, whereas no correlation was observed under pine. We conclude that weathering reactions or stoichiometry differed between vegetation types.Corresponding author     

The effects of liming an Adirondack lake watershed on downstream water chemistry

Calcite treatment of chronically acidic lakes has improved fish habitat, but the effects on downstream water quality have not previously been examined. In this study, the spatial and temporal effects of watershed CaCO₃ treatment on the chemistry of a lake outlet stream in the Adirondack Mountains of New York were examined. Before CaCO₃ treatment, the stream was chronically acidic. During spring snowmelt before treatment, pH and acid-neutralizing capacity (ANC) in the outlet stream declined, and NO ₃ ^– and inorganic monomeric aluminum (Al_IM) concentrations increased sharply. During that summer, SO ₄ ^– and NO ₃ ^– concentrations decreased downstream, and dissolved organic carbon (DOC) concentrations and ANC increased, in association with the seasonal increase in decomposition of organic matter and the attendant SO ₄ ^– -reduction process. A charge-balance ANC calculation closely matched measured downstream changes in ANC in the summer and indicated that SO ₄ ^– reduction was the major process contributing to summer increases in ANC. Increases in Ca²⁺ concentration and ANC began immediately after CaCO₃ application, and within 3 months, exceeded their pretreatment values by more than 130 eq/L. Within 2 months after treatment, downstream decreases in Ca²⁺ concentration, ANC, and pH, were noted. Stream mass balances between the lake and the sampling site 1.5 km downstream revealed that the transport of all chemical constituents was dominated by conservative mixing with tributaries and ground water; however, non-conservative processes resulted in significant Ca²⁺ losses during the 13-month period after CaCO₃ treatment. Comparison of substrate samples from the buffered outlet stream with those from its untreated tributaries showed that the percentage of cation-exchange sites occupied by Ca²⁺ as well as non-exchangeable Ca, were higher in the outlet-stream substrate than in tributary-stream substrate. Mass-balance data for Ca²⁺ H⁺, Al_IM, and DOC revealed net downstream losses of these constituents and indicated that a reasonable set of hypothesized reactions involving Al_IM, HCO ₃ ^– , Ca²⁺, SO ₄ ^– NO ₃ ^– , and DOC could have caused the measured changes in stream acid/base chemistry. In the summer, the sharp decrease in ANC continued despite significant downstream decreases in SO₄ ^2– concentrations. After CaCO₃ treatment, reduction of SO ₄ ^– was only a minor contributor to ANC changes relative to those caused by Ca²⁺ dilution from acidic tributaries and acidic ground water, and Ca²⁺ interactions with stream substrate.     

The control of shoot tip necrosis in Pistacia vera L. in vitro

The effect of increased concentrations of calcium (Ca) (3–24 mM) and boron (B) (100–800 M) in the medium was studied on the occurrence of shoot tip necrosis (STN) in cultures of Pistacia vera L. STN was significantly reduced by application of Ca or B, however media with more than 200 M boron had reduced shoot multiplication. Ca (12–24 mM) supplied as calcium chloride reduced STN without any adverse effect on shoot multiplication or elongation, whereas calcium acetate reduced elongation. It is concluded that STN is a physiological mineral disorder associated with Ca and/or B deficiency in the meristematic regions of actively growing shoots. Application of Ca (up to 24 mM) as calcium chloride to the medium was the best treatment for the control of STN. Reduction of humidity or increased aeration in the culture jars did not have any significant effect on the occurrence of STN.Abbreviations MS Murashige and Skoog medium - STN Shoot tip necrosis     

Distribution of mineral nutrient from nodal roots of Trifolium repens: Genotypic variation in intra-plant allocation of 32P and 45Ca

To assess genotypic variability in nutrient supply of shoot branches, the distribution of ³²P and ⁴⁵Ca exported from a source nodal root (24-h uptake period) was measured within a genotype of a large-leaved (Kopu) and a small-leaved (Tahora) cultivar of Trifolium repens. Source-sink relationships of plants were modified by root severance, defoliation, and shade treatments. In control plants of both genotypes distribution of ³²P and ⁴⁵Ca closely followed the pathways that could be predicted from the known phyllotactic constraints on the vascular system. As such there was little allocation of radioisotopes (3.1% and 2.5% of exported ³²P and ⁴⁵Ca, respectively) from the source root to branches on the apposite side of the parent axis (far-side branches). However, genotypic differences in nutrient allocation were apparent, when treatments were imposed to alter intra-plant source-sink relationships. In the large-leaved genotype, the imposed treatments had minor effects on the allocation to far-side branches: whereas, in the small-leaved genotype, root severance and defoliation treatments increased lateral transport to far-side branches to 30% (³²P) and 10% (⁴⁵Ca) of exported radioisotopes. Genotypes with low (8–9) and high (12–13) numbers of vascular bundles were selected from within the large-leaved cultivar. Distribution of ³²P was then measured after plants had been pre-treated by removal of all far-side roots two days prior to labelling. Genotypes with low vascular bundle number allocated 20% and those with high vascular bundle number 3.2% of exported ³²P to far-side branches. It was concluded (1) that genotypic variation exists within T. repens for potential to alter intra-plant allocation of mineral nutrients, in response to treatments that modify source-sink relationships within plants; and (2) that this variation is correlated with differences among genotypes in the organisation of the vasculature of their stolons.     

Induction and ionic basis of slow wave potentials in seedlings of Pisum sativum L.

Slow wave potentials (SWPs) are transient depolarizations which propagate substantial distances from their point of origin. They were induced in the epidermal cells of pea epicotyls by injurious methods such as root excision and heat treatment, as well as by externally applied defined steps in xylem pressure (Px) in the absence of wounding. The common principle of induction was a rapid increase in Px. Such a stimulus appeared under natural conditions after (i) bending of the epicotyl, (ii) wounding of the epidermis, (iii) rewatering of dehydrated roots, and (iv) embolism. The induced depolarization was not associated with a change in cell input resistance. This result and the ineffectiveness of ion channel blockers point to H(+)-pumps rather than ion channels as the ionic basis of the SWP. Stimuli such as excision, heat treatment and pressure steps, which generate SWPs, caused a transient increase in the fluorescence intensity of epicotyls loaded with the pH-indicator DM-NERF, a 2',7'-dimethyl derivative of rhodol, but not of those loaded with the pH indicator 2',7'bis(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF). Matching kinetics of depolarization and pH response identify a transient inactivation of proton pumps in the plasma membrane as the causal mechanism of the SWP. Feeding pump inhibitors to the cut surface of excised epicotyls failed to chemically simulate a SWP; cyanide, azide and 2,4-dinitrophenol caused sustained, local depolarizations which did not propagate. Of all tested substances, only sodium cholate caused a transient and propagating depolarization whose arrival in the growing region of the epicotyl coincided with a transient growth rate reduction.     

Suppression of programmed neuronal death by a thapsigargin-induced Ca2+ influx

Rat sympathetic neurons undergo programmed cell death (PCD) in vitro and in vivo when they are deprived of nerve growth factor (NGF). Chronic depolarization of these neurons in cell culture with elevated concentrations of extracellular potassium ([K⁺]_o) prevents this death. The effect of prolonged depolarization on neuronal survival is thought to be mediated by a rise of intracellular calcium concentration ([Ca²⁺]_i) caused by Ca²⁺ influx through voltage-gated channels. In this report we investigate the effects of chronic treatment of rat sympathetic neurons with thapsigargin, an inhibitor of intracellular Ca²⁺ sequestration. In medium containing a normal concentration of extracellular Ca²⁺ ([Ca²⁺]_o), thapsigargin caused a sustained rise of intracellular Ca²⁺ concentration and partially blocked death of NGF-deprived cells. Elevating [Ca²⁺]_o in the presence of thapsigargin further increased [Ca²⁺]_i, suggesting that the sustained rise of [Ca²⁺]_i was caused by a thapsigargin-induced Ca²⁺ influx. This treatment potentiated the effect of thapsigargin on survival. The dihydropyridine Ca²⁺ channel antagonist, nifedipine, blocked both a sustained elevation of [Ca²⁺]_i and enhanced survival caused by depolarization with elevated [K⁺]_o, suggesting that these effects are mediated by Ca²⁺ influx through L-type channels. Nifedipine did not block the sustained rise of [Ca²⁺]_i or enhanced survival caused by thapsigargin treatment, indicating that these effects were not mediated by influx of Ca²⁺ through L-type channels. These results provide additional evidence that increased [Ca²⁺]_i can suppress neuronal PCD and identify a novel method for chronically raising neuronal [Ca²⁺]_i for investigation of this and other Ca²⁺-dependent phenomena. © 1995 John Wiley & Sons, Inc.     

The Ca2+ influx induced by β-amyloid peptide 25–35 in cultured hippocampal neurons results from network excitation

Although a neurotoxic role has been postulated for the β-amyloid protein (βAP), which accumulates in brain tissues in Alzheimer's disease, a precise mechanism underlying this toxicity has not been identified. The peptide fragment consisting of amino acid residues 25 through 35 (βAP25-35), in particular, has been reported to be toxic in cultured neurons. We report that βAP25-35, applied to rat hippocampal neurons in culture, caused reversible and repeatable increases in the intracellular Ca²⁺ concentration ([Ca²⁺]_i), as measured by fura 2 fluorimetry. Furthermore, βAP25-35 induced bursts of excitatory potentials and action potential firing in individual neurons studied with whole cell current clamp recordings. The βAP25-35–induced [Ca²⁺]_i elevations and electrical activity were enhanced by removal of extracellular Mg²⁺, and they could be blocked by tetrodotoxin, by non-N-methyl-D -aspartate (NMDA) and NMDA glutamate receptor antagonists, and by the L-type Ca²⁺ channel antagonist nimodipine. Similar responses of bursts of action potentials and [Ca²⁺]_i increases were evoked by βAP1-40. Responses to βAP25-35 were not prevented by pretreatment with pertussis toxin. Excitatory responses and [Ca²⁺]_i elevations were not observed in cerebellar neuron cultures in which inhibitory synapses predominate. Although the effects of βAP25-35 depended on the activation of glutamatergic synapses, there was no enhancement of kainate- or NMDA-induced currents by βAP25-35 in voltage-clamp studies. We conclude that βAP25-35 enhances excitatory activity in glutamatergic synaptic networks, causing excitatory potentials and Ca²⁺ influx. This property may explain the toxicity of βAP25–35. © 1995 John Wiley & Sons, Inc.     

Spontaneous calcium transients regulate neuronal plasticity in developing neurons

Calcium ions play critical roles in neuronal differentiation. We have recorded transient, repeated elevations of calcium in embryonic Xenopus spinal neurons over periods of 1 h in vitro and in vivo, confocally imaging fluo 3-loaded cells at 5 s intervals. Calcium spikes and calcium waves are found both in neurons in culture and in the intact spinal cord. Spikes rise rapidly to approximately 400% of baseline fluorescence and have a double exponential decay, whereas waves rise slowly to approximately 200% of baseline fluorescence and decay slowly as well. Imaging of fura 2-loaded neurons indicates that intracellular calcium increases from 50 to 500 nM during spikes. Both spikes and waves are abolished by removal of extracellular calcium. Developmentally, the incidence and frequency of spikes decrease, whereas the incidence and frequency of waves are constant. Spikes are generated by spontaneous calcium-dependent action potentials and also utilize intracellular calcium stores. Waves are produced by a mechanism that does not involve classic voltage-dependent calcium channels. Spikes are required for expression of the transmitter GABA and for potassium channel modulation. Waves in growth cones are likely to regulate neurite extension. The results demonstrate the roles of a novel signaling system in regulating neuronal plasticity, that operates on a time scale 10⁴ times slower than that of action potentials. © 1995 John Wiley & Sons, Inc.